This isn't really ideal in plugins as you'll see below because they are much more prone to becoming unsupported and they by their nature extend rather than integrate. Having integrated lifestream support would encourage extensions to build upon the concept. By using wp_posts (not a good idea to do as a plugin since version changes can cause havoc) it becomes much easier to integrate and preserve data.

Here's what I've been thinking:
- Popular services already programmed in, easy to extend. RSS is pretty standard these days making it easy to support new services. Enter twitter username. Use a more obscure twitter clone? Just drop in the rss feed.
- All data saved into wp_posts table as new entries. This make it easy to work with. It's as simple for template and plugin developers as a post or page.
- Expands shortened url's if found, and (optionally) saves images locally for performance and so that if a service were to disappear from the web (think tr.im's threat), your life stream is preserved in your wordpress database and filesystem. Your WordPress instance becomes an online record of your activities and contributions to the social web.
- Option to show all data on your blog, or just regular blog posts (ala "classic mode"), separating life stream support out for a separate page (/stream for example). This would allow people to keep their blog the way it is and have a separate life stream page, or to just make the entire thing a life stream. RSS feed treated respectively for those afraid to bombard their subscribers with all their diggs and retweets, etc.

Even further:
- WordPress can be extended via plugin to support various services to allow you to contribute rather than just capture and save data. Think twitter client in WordPress.

There are plugins that do parts of this, twitter-tools, lifestream, etc. But none of them take an integrated approach and allow for the option to combine and separate your content as is really necessary. While clever, they aren't really long term solutions for those who really care about their data.

My idea would effectively allow those who like keeping their blog and social site usage separate to do so, those who want to integrate to do so, and those who want to keep a copy of their social presence on their WordPress instance, but not tightly integrate in their blog to do so as well. Everyone wins.